1. Field of the Invention
The present invention relates to a method of employing a surfactant which is provided in order to assist in the release from each other of mold components of a multi-part mold employed in the molding of polymeric articles; for instance, such as a hydrophilic contact lens, upon completion of the molding process for the polymeric articles.
The method pursuant to the invention also contemplates the application of the surfactant in the form of a film or coating on surface portions of one of the mold components in order to facilitate the disengagement between the mold components during demolding, and the removal of excess polymeric molding material adhesively deposited on surfaces thereon; which, for example, may be in the shape of rings formed externally of the respective mold cavities for the contact lenses, and which are formed through the displacement of polymeric material from the mold cavity.
Moreover, pursuant to a further aspect of the invention, there is provided a novel and unique apparatus which is adapted to effectuate the application of a coating or film containing a surfactant to surfaces of a mold component for molding hydrophilic polymer articles, especially such as hydrophilic contact lenses or the like. Accordingly, any excess molding material encountered during the molding of the contact lenses which is expelled from the mold cavities and resultingly adhesively deposited in the shape of rings on surface portions between the mold components located externally about the mold cavities, can be easily and cleanly removed upon heat separation of the mold components in view of the presence of the surfactant coating or film. This also facilitates the essentially undamaged removal of the molded articles from the mold cavities during separation of the components thereof upon completion of the molding process for the articles.
An extensive array of methods is currently employed in this technology in carrying out the manufacture of hydrophilic polymer articles, such as hydrophilic contact lenses, through the implementation of a considerable variety of processes; among which there are included spin casting, static cast molding, lathing, combinations of casting and lathing, and particularly frequently, through the use of two-part molds. In the utilization of separable two-part molds for the molding of hydrophilic contact lenses, upon completing the molding step it is of critical importance to be able to release the hydrophilic contact lenses from their surface engagement with and adherence to the mold parts, or at least one of the mold halves, without causing any damage to the contact lenses upon separating the mold parts, which would render the lenses unusable. Various concepts employed for effectuating the release of hydrophilic contact lenses from surfaces of molds, particularly two-part molds which are intended to be physically separated, are through the employment of laser or infrared heat mold part separating devices, the use of external heat and/or the application of heated gas, hot air or steam. However, all too frequently, damage is sustained by the contact lenses during removal thereof from the molds in that the material of the mold parts may exhibit a tendency to adhere and inhibit easy release upon effectuating separation of the mold parts. This potential damage which generally renders the contact lenses unable to meet necessary quality and/or inspection standards may consist of tears, nicks or other surface defects formed in the contact lenses, prevalent of which there may be the so-called "piece of material" (POM) defects caused by excess polymer debris or particles produced during molding permanently adhering to the contact lenses upon mold separation.
Generally, such mold parts for hydrophilic contact lenses consist of a curvilinear (concave/convex) center or front curve mold part adapted to form, in cooperation with a mating base mold curve part, a two-part mold for the contact lenses. Hydrophilic contact lenses of the type considered herein are usually constituted from a hydrophilic polymer, preferably a HEMA-based polymer (hydroxyethylmethacrylate), although other suitable monomers may comprise hydroxy ethyl acrylate (HEA), hydroxypropyl methacrylate, hydroxy propyl acrylate and hydroxy trimethyl ethylene acrylate, among numerous other applicable materials.
The components of the mold within which the hydrophilic polymer contact lenses are molded may consist of suitable pallets each including a plurality of cavities receiving mold parts having female and/or male base surfaces or curvilinear wall portions for the formation of the curved contact lenses and an annular rim portion extending thereabout. The molds as described, for instance in the molding technology, may be constituted from suitably selected conventional plastic materials, whereby the hydrophilic polymer articles; i.e. the contact lenses, will possibly adhere to one of the mold parts through the presence of mold parts having different properties or being formed of different materials. The mold parts may consist of plastic materials such as polyethylene, polypropylene, polystyrene, polyvinyl chloride among other suitable plastic mold materials, as is known in the technology.
In order to remove the molded hydrophilic contact lenses from the respective mold cavities, separation of the mold halves or portions is implemented, and the formed contact lenses may then be removed from the mold cavities. Due to excess portions of the polymeric material of the hydrophilic contact lenses which are expelled from the mold cavities of the cooperating mold components, and which form ring-shaped elements of the HEMA-based polymer from which the contact lenses are made surrounding the exteriors of the mold cavities, and exhibit tendencies to strongly adhere to the mold surfaces on which the rings are deposited. These rings make separating of the mold cavities difficult, resulting in mold breakage and damage to the lenses. Further, the rings, or fragments thereof, become uncontrollable debris in automated production lines, contaminating both the production line and the final lens package. Moreover, the foregoing can also conceivably cause a perfectly "good" contact lens to be rejected after demolding due to the formed HEMA-ring or a portion thereof remaining on the front curve of the mold. This results in uneconomical production conditions being encountered during the manufacture of such contact lenses.
2. Discussion of the Prior Art
In order to implement the high-speed and mass-produced molding of such hydrophilic contact lenses, there have been developed two-part molds incorporating pallet-supported mold structures; for example, as disclosed in U.S. Pat. No. 4,640,489 to Larsen, and wherein methods of forming shaped polymeric hydrogel articles, such as hydrophilic contact lenses, are also elucidated in the disclosures of U.S. Pat. Nos. 4,680,336 and 5,039,459 to Larsen et al., wherein the last-mentioned two patents are assigned to the common assignee of the present application and the disclosures of which are incorporated herein by reference.
The release of hydrophilic contact lenses from adherent mold surfaces subsequent to the completion of the contact lens molding process can be facilitated or improved upon, as is set forth in the disclosure of U.S. Pat. No. 5,264,161 to Druskis et al. In that instance, surfactants are introduced in solution into a hydration bath employed in the molding cavities for molding the hydrophilic polymeric structures or contact lenses. The surfactant which is dispersed in the hydration bath in concentrations not exceeding 10% by weight aids in facilitating release of the lenses from adherent contiguous mold surfaces being separated, with the function of such surfactant being to reduce the surface tension properties of water or liquids, and to thereby reduce the level of adherence between components consisting, on the one hand, of the contact lenses and, on the other hand, the mold surfaces which become adherent during molding. Numerous types of surfactants are disclosed in this patent publication, particularly such as polymeric surfactants including polyoxyethylene sorbitan mono-oleates, and which are especially suitable for releasing in an undamaged state any hydrophilic polymer articles from adherent mold surfaces which are constituted of plastic materials. Nevertheless, the utilization of surfactants in suspension in a liquid hydration bath does not provide the desired control over the release of the hydrophilic polymer articles or contact lenses from the mold surfaces prior to hydration in the bath and, in particular, does not readily afford an easy removal of HEMA-based rings of molding material formed in adherence with mold surfaces extending about the mold cavities, or a more effortless separation between the mold component, whereby the rate of damage being encountered by the contact lenses upon mold separation and removal from the molds is still unacceptably high and is not readily controllable to a degree affording a satisfactory economic manufacturing environment.